The present invention relates to the fields of dynamic tomography and computed tomography. More particularly, its pertains to a method for high resolution digital image registration within a computerized dynamic tomography system.
Accurate and detailed visual information about the internal structure of an object is extremely valuable in a variety of applications. In the practice of medicine for example, visual examination of internal organs or bones is necessary to properly diagnose many ailments or to prepare for surgery. Non-invasive techniques, such as x-ray examination, often provide the only means of obtaining such visual information. As another example, quality control analysis of manufactured products requires inspection of internal parts.
Several techniques have been employed to obtain visual information about the internal structure of an object, without opening the object. Generally, penetrating radiation, such as x-rays or gamma rays, are directed at an object and the radiation which is transmitted through the object is recorded either on radiographic film or with electronic radiation detectors employing scintillation crystals. According to one known technique, "Computed Tomography" (CT), a radiation source is mounted in rotatable relation to a row of radiation detectors and an object is placed therebetween. The radiation source is then rotated to expose a section of the object to radiation from several angles and radiation measurements made by the radiation detectors are processed by a computer to generate a two-dimensional "slice", representative of the internal structure of the exposed section of the object.
This limitation of a single slice for each section of the object presents several problems if more than one slice is desired, as, for example, in generating a three-dimensional internal image of the object. First, the object must be moved discrete distances at discrete time intervals, corresponding to the "thickness" of each slice and the amount of time required to rotate the radiation source, thus requiring a complicated and expensive mechanical system to move the object in coordination with the rotating radiation source. Second, the object must be exposed to additional radiation for each additional slice, resulting in increased radiation dosage in proportion to the number of slices desired. Additionally, the amount of time required to complete the procedure is prolonged by each slice.
According to another known technique, "Dynamic Tomography", see, e.g., Richards, U.S. Pat. No. 4,167,672, a set of radiographs of an object is produced by exposing the object to radiation from a plurality of angles and recording each exposure on a separate piece of radiographic film. The set of radiographs can be superimposed in a stack for viewing, and by shifting alignment can produce a virtual focus of any image plane parallel to the plane of each film. This technique solves the single slice problem presented by computed tomography because the entire internal structure of the object can be displayed with a small number of radiographs. However, several other problems are presented. First, a complicated mechanical viewing system is required to achieve proper alignment while shifting the stack of radiographs. Second, the accuracy of alignment is restricted by the limitations of the physical components of a mechanical viewing system and the film itself. Third, image enhancement and manipulation are not possible with a stack of radiographs. Additionally, image planes non-parallel to the plane of each film cannot be adequately displayed by mechanical means.
A computerized dynamic tomography system, as described in U.S. patent application Ser. 172,609 now U.S. Pat. No. 5,051,904 can display any image plane or series of image planes of an object, for any desired angle, from a small number of radiographic images of the object. A radiation source exposes the object to penetrating radiation at a plurality of partial rotations of the object on a platform assembly. Reference marks are maintained in a constant position adjacent to a radiographic film to facilitate registration of the object. A separate radiographic film records the transmitted radiation at each partial rotation and each film is digitized by a digitizing camera. The digitized images are then supplied to a computer which registers each object image using the digitized reference mark image and with minimal assistance from a human operator.
The operator selects a level or series of levels desired for viewing and the computer displaces and selectively combines pixel values of the digitized images to produce selected images of the internal structure of the object. Interactive enhancement and manipulation of images is provided and compression of images is done to minimize the memory requirements of the invention.
The present invention provides an accurate method of locating the center of digital images by a computerized dynamic tomography system. Specifically, the present invention comprises a method for locating the center of a reference mark image within a high resolution digital image containing the reference mark image and image defects, represented by pixel values, in which: The pixel values defining an image defect are altered to reduce the range of pixel values representing the image defect; the pixel values defining the digital image are altered to reduce the range of pixel values representing the digital image; and a center of the resulting reference mark image is located.
In a preferred embodiment, the present invention is a method of enhancing the digital image of a reference mark and providing for accurate location of the center of a reference mark. Reference mark enhancement is a two step process requiring the removal of internal image defects contained within a reference mark image and enhancement of the pixel value contrast. The presence of internal defects and vague edges within a reference mark decreases the accuracy of known methods for locating the center of the mark. If the pixel value contrast is increased without removal of internal defects, some of the internal defects are enhanced rather than eliminated. If the center of a reference mark is located without enhancement of pixel values, accurate determination of the center of reference marks with poor edge quality is difficult. The present invention provides for accurate location of the center of reference marks with internal defects and vague edges by producing an image of a reference mark in which the interior is uniformly bright and the edge is sharp.